This paper discusses an on-chip Red Blood Cell (RBC) deformability checker with vision analyzer. The system is composed of a PDMS chip including three microfluidic channels, a microscope for appropriately enlarging RBC, and a high-speed vision for observing both the cell size and behaviors in the channel, respectively. We particularly focus on how to analyze the deformability of single cells by the proposed vision analyzer. The analyzer can detect the diameter and velocity of each cell inside and outside the test channel. Based on the information, we can obtain a deformability map which shows the relationship between cell deformation and transit velocity. The results obtained by the proposed vision analyzer are compared to the manual evaluation with a tuning parameter based on gray level control. The comparison results show that the error is less than 5% between vision-based and manual evaluations.